The present invention relates to remote measurement and more particularly but not exclusively to remote, that is to say non-contact, methods of measuring resistivity. and/or thickness.
In the manufacture of silicon chips the outer surface of the silicon substrate is often coated with a layer known as an epitaxial layer (or epilayer). The purpose of this layer is to give additional quality to the substrate because the epitaxial layer can be made more close to a perfect crystal. The epitaxial layer has dopant which is added during the plasma phase and is of high resistivity relative for example to metal. The substrate has qualities of its own and also contains features in the sub-micron region. The epitaxial layer covers defects in the substrate.
A typical thickness of an epitaxial layer is 5-50 xcexcm. In order to control the manufacturing process of such thin layers it is necessary to take measurements dynamically during the process. In particular it is necessary to monitor the resistivity. Measurements should preferably not interfere with the manufacturing process itself
There is known a method of measuring the resistivity of surfaces by radiating the surface with microwaves. The reflections of the microwaves from the surface are affected by the resistivity of the surface so that, following suitable calibration, a measurement of resistivity may be made.
Measurements of this sort are appropriate for homogeneous blocks of material. They cannot, however, be carried out on very thin layers.
The present invention seeks to enable the measurement of properties of an upper thin layer on the surface of a block of material, that is to say to measure such properties as resistivity and thickness. It is possible to eliminate the difficulty of having reflections from different material layers by setting a receiver to receive signals during a very short time window. Reflections from anything but the top layer do not have time to arrive during this window.
According to the present invention there is provided apparatus for non-contact measurement of resistivity of a surface layer of a wafer, typically a semiconductor wafer with a coating which is an epitaxial layer. The apparatus comprises a source of high frequency radiation, for irradiating the wafer, a detector of high frequency radiation, positioned to detect radiation from the source reflected from the wafer, and a high speed switching device, operatively connected to both said detector and said source, and operable to switch on the detector in time to receive radiation reflected from the surface layer of the wafer. The high frequency radiation is preferably in the sub-millimeter wavelength range. The time interval between the switching on of the detector and switching off of the detector may be in the order of 10xe2x88x9213 seconds.
In an embodiment the high speed switching device is an optical switching device, and may comprise a laser or other light source, a beam splitting device and an array of mirrors arranged to form a delay path for one of the two beams emerging from the beam splitting device. One beam heads directly for the short r.f. source and generates a pulse. The delayed beam goes to the detector and serves as an on signal. Preferably the array of mirrors is adjustable to enable the delay path to be altered.